1. Field of the Invention
The present invention relates to a method of manufacturing a hollow shaft, and a mandrel for holding a relatively long hollow shaft blank.
2. Description of the Related Art
According to one conventional process of manufacturing a relatively long hollow shaft, an axial deep hole is drilled in a shaft blank in the form of a solid cylinder by a gun drill, and then a thin hole is drilled in the shaft blank by a drill, so that a through hole is formed in the shaft blank. Then, a mandrel in the form of a solid cylinder is inserted into the through hole in the hollow shaft blank. The mandrel has an outside diameter which is substantially the same as the inside diameter of the through hole in the hollow shaft blank, and is longer than the hollow shaft blank. When the mandrel is inserted into the through hole in the hollow shaft blank, the mandrel holds the hollow shaft blank substantially concentrically therewith, and has its opposite ends held between the centers of a lathe or the like. Thereafter, while the hollow shaft blank supported on the mandrel is rotating, the hollow shaft is machined at its outer circumferential surface, thereby producing a hollow shaft.
Details of the manner in which the hollow shaft blank is supported on the mandrel will be described below. As shown in FIG. 8 of the accompanying drawings, a mandrel 50 comprises a main stem 53 whose outer circumferential surface is held almost in its entirety against the cylindrical surface which defines a through hole 52 in a hollow shaft blank 51, and an extension stem 54 extending coaxially from an end of the main stem 53. The through hole 52 in the hollow shaft blank 51 includes a smaller-diameter portion 55 at an end thereof in which the extension stem 54 is inserted. A radial step defined by the smaller-diameter portion 55 in the hollow shaft blank 51 is engaged by a radial step defined between the main and extension stems 53, 54 of the mandrel 50, and a nut 56 is threaded over the outer end of the extension stem 54 which projects out of the hollow shaft blank 51, thereby securely holding the mandrel 50 in the hollow shaft blank 51. For machining the outer circumferential surface of the hollow shaft blank 51, the mandrel 50 held between the centers of a lathe is rotated about its own axis to rotate the hollow shaft blank 51 about its own axis.
Since the mandrel 50 is removably inserted in the through hole 52 in the hollow shaft blank 51, the mandrel 50 is slightly smaller in diameter than the through hole 52 in the hollow shaft blank 51. Therefore, there is a very small clearance of about 30 .mu.m between the outer circumferential surface of the mandrel and the cylindrical surface which defines the through hole 52 in the hollow shaft blank 51. Due to this clearance, it is impossible to keep the hollow shaft blank 51 and the mandrel 50 concentric with each other highly accurately, and hence to keep the concentricity of the hollow shaft blank 51 at a high level, i.e., to eliminate a misalignment between the axes of inner and outer circumferential surfaces of the hollow shaft blank 51. When the hollow shaft blank 51 is rotated through the mandrel 50, the hollow shaft blank 51 tends to vibrate radially, and cannot be machined highly accurately at its outer circumferential surface.
Hollow shafts of this type are used as aircraft gas turbine shafts, for example, and should preferably have a very high level of concentricity ranging from 10 to 20 .mu.m. Those hollow shafts are often made of difficult-to-machine materials including nickel-based super alloys and stainless-steel-based alloys, for example. Therefore, it is relatively difficult to machine those hollow shafts.
Hollow shafts that are manufactured in the manner described above have a relatively low level of concentricity ranging from 70 to 120 .mu.m, and cannot be machined with accuracy if they are made of difficult-to-machine materials. Accordingly, the hollow shafts thus manufactured are not suitable for use as shafts of high dimensional accuracy, e.g., aircraft gas turbine shafts.